In recent years, a connector assembly is used to interconnect two printed wiring boards in electronic devices such as a cellular phone or a digital camera.
As shown in FIG. 9, there has been conventionally known a method of mechanically and electrically interconnecting a first circuit board 8 and a second circuit board 9 having surfaces at least partially facing each other (see, e.g., Japanese Patent No. 4059522).
Electrically conductive projections 81a are formed on one surface of the first circuit board 8. The second circuit board 9 includes a flexible insulation film 92 having a plurality of substantially cross-shaped in-plane slit portions 9b into which the conductive projections 81a of the first circuit board 8 are to be inserted. Pad portions 9c whose outer circumference has a circular shape as shown in FIG. 10 are formed around the slit portions 9b. The pad portions 9c come into conduction with the circuit patterns 9a of the second circuit board 9 and the conductive projections 81a of the first circuit board 8. The second circuit board 9 further includes a reinforcing plate 9e made from, e.g., a substantially flat metal plate or insulating plate and bonded to the other surface of the second circuit board 9. The reinforcing plate 9e has a plurality of insertion holes 9d formed in corresponding portions to the slit portions 9b. 
The conductive projections 81a of the first circuit board 8 are inserted into the slit portions 9b at one side of the second circuit board 9. Thus, the conductive projections 81a are fitted to and coupled with the slit portions 9b to come into conduction with the pad portions 9c, whereby the first circuit board 8 and the second circuit board 9 are electrically and mechanically connected to each other.
However, if the pad portions 9c have a circular shape as shown in FIG. 10, stresses are concentrated on the tip ends 9f of the pad portions 9c at which the pad portions 9c and the conductive projections 81a make contact with each other. Therefore, the pad portions 9c may possibly be damaged when engaging and separating the first and second circuit boards 8 and 9 (see FIGS. 11A and 11B).